The present invention relates generally to patellar components that are designed to form a patella portion (or knee cap) that replaces a part of a natural patella or knee cap, and particularly to patellar components that are designed to cooperate and articulate against a femoral component of a total knee prosthesis. Also provided are methods for implanting the described patellar components.
Joint replacement, and particularly knee replacement, has become increasingly widespread. Various knee prostheses and procedures have been developed to treat the debilitating effects of knee joint deterioration (e.g., such as that caused by arthritis, injury, or disease). A fairly common procedure used to repair a patient's knee is total knee replacement, in which the tibia is resected and replaced with a tibial component, and the femur is resected and replaced with a femoral component. In some instances, the surgeon will also replace the articulating surface on the posterior aspect of the patella where it interfaces with the femoral component, which can help improve the results of total or partial knee replacements.
The primary function of the patella is to increase the efficiency of the quadriceps muscles and to serve as a connection between the quadriceps tendon and the patellar tendon. The patella has a ridge on its posterior side which slides in a groove between the femoral condyles, referred to as the patellar track. The patella, patellar track, and condyles act together as a low friction pulley and lever for the quadriceps tendon.
As shown in FIG. 1, during knee replacement surgery, a prosthetic patellar component 2 can be affixed to the natural patella 4 so that its posterior side 6 contacts the femoral component 8 during flexion and extension of the knee. The patellar component 2 then tracks the trochlear groove 9 of the femoral component (the line that separates the femoral condyles) during flexion and extension of the knee.
In order to implant the patellar component 2, a part of the natural patella bone is removed, and an implant is secured thereto. Some implants are “inset,” meaning that a shallow hole is drilled into the bone using a counterbore so that the implant lies about flush with the bone. Other types are “onset,” which means that the back of the patella is planed off and the implant is placed on top of the flat bone. The invention described in this application transcends these types of implants and can be used for both.
The maximum range of motion for a natural knee is about 150-160 degrees. By contrast, most knee replacements achieve only about 110-120 degrees of flexion. Some of this gap can be attributed to scarring within the knee joint and other physical conditions, but the remainder can be attributed primarily to failure of current implants to provide the proper prosthetic component geometries that take into consideration the natural kinematics of the knee.
Thus, despite the relative success of some products on the market, many patellar components tend to fail after about five to fifteen years. Part of the reason they fail is due to excessive wear at certain regions of the component or loosening at the bone/component interface. For example, with a dome patellar component, one reason for failure is the downward forces of the femoral component acting on the patellar component during flexion can cause the pegs that extend from the patellar component and attach the component to the natural patella to weaken and break. Shear forces that are applied directly to the implant change during knee motion, and they increase as the knee is moved deeper into flexion. For example, at different angles of flexion, the contact stresses across the patello-femoral interface move outward towards the periphery of the interface and increase as knee flexion increases. During knee flexion, the patella itself flexes and the contact point moves superiorly on the component articular surface. At this location, the contact force (normal to the articular surface of the component) creates backside compressive forces and backside shear forces.
For example, when a person is standing upright, the normal line of pressure that the femoral component 8 exerts on a button or dome patellar component 2 points at or near the center of the component dome. An example of the line of pressure is shown in FIG. 2, by arrow A. During flexion (e.g., as a person squats), the flexion of the knee increases and causes the contact point between the femoral component and the patellar component to roll deeper in the trochlear groove and into the condyles. The line of pressure also swings up from the center of the dome toward the upper part of the component. In full flexion, the line of pressure is no longer normal to the natural patella, but is directed downward, pointing down toward an upper portion of the patellar component, as shown by arrow B in FIG. 3. In full flexion, the force vector applies pressure at an upper portion of the patella. It essentially tries to “push” the button patella component off of the patellar bone surface. This force will be referred to as shear force. Part of the reason this occurs is because of the button patella's axis-symmetric dome-like shape.
A useful analogy for illustrating shear force is to consider a dome-shaped paperweight on a desk top. If someone applies pressure at the center of the dome, the paperweight stays in place. This is analogous to a person standing upright, with the force (i.e., compressive force) of the femoral component being directed at the center of the patellar dome. Referring back to the paperweight example, if a person swings the pressure point normal to the surface and toward an upper part of the paperweight (e.g., toward one of the edges), the paperweight will slide along the surface of the desk. This is analogous to the knee in flexion, where the force of the femoral component is directed toward an upper part of the patellar component. In this position, the pegs that attach the patellar component to the patella are particularly stressed. In fact, a number of artificial patella failures are due to peg failures, and these types of forces can be painful for the patient. This force is referred to as shear force, and embodiments of this invention seek to avoid or decrease shear force.
Part of the reason that shear force causes a problem with artificial patellar components and not with a natural patella is because artificial patellar components tend to be shaped like an arc or a dome, whereas the natural patella is more linear shaped. Because a natural patella is flatter, as the patella rotates during flexion and the contact vector(s) travel up the patella (note that when contact is with the condyles, there are two contact vectors, one from each condyle, and when contact is with the trochlear groove, there is a single contact vector), the force vector(s) from the femoral component still remain somewhat normal (or perpendicular) to the patella, as opposed to pointing down at an angle at the dome of a patellar component.
Under ideal conditions, anatomically-shaped patella components exhibit lower interface shear forces due to their contact condition and shape. However, they can be particularly sensitive to high contact stress edge loading due to mal-rotation or positioning during surgical implantation. For example, many of the highly-conforming “anatomic” patella designs offered have different levels of constraint between the trochlear groove articulation area and the intracondylar articulation area. The constraint difference is magnified in the anatomic design compared to a button (or dome) design because the anatomic design interfaces with more of the femoral surface. This magnified constraint difference causes the anatomic patellar components to shift their alignment (equilibrium) with respect to the femoral component (particularly in rotational degrees of freedom) when moving from one area to the other, which can be perceived as an instability, pain, or a clunk to the patient. The shift seems to be more aggressive during ascent, when the patella is moving from the intracondylar area into the trochlear groove. The design challenge faced is to design surfaces of patellar components that minimize the shift in constraint, while still reducing the component/bone shear forces. Such designs will hopefully reduce patient pain and prolong the life of components.
Accordingly, embodiments of the invention minimize shear force load between a patella component and the patella bone, but still provide a component that is not as sensitive to surgical mal-implantation or functional kinematics as highly-conforming “anatomically-shaped” patellas are.
Another challenge experienced by patellar component manufacturers is to design a component that lessens the shear force issues described, but that also accommodates variations introduced by surgical inaccuracy and patient anatomical variation. For example, it is difficult to position a patellar component so that it precisely matches the orientation of the patello-femoral groove geometry on the femoral component. Accordingly, most designs on the market use an axis-symmetric configuration for the bearing surface of the patellar component (as shown in FIGS. 1-3), which results in a low conformity between the patellar component part and the femoral component. With such a design, the surgeon does not need to exactly and precisely position the component in order for it to function. However, such components cause the above-described shear force problems due to their shape. Other designs have attempted to make patellar components flatter or more concave, so that they more closely approximate a natural patella, but as the components are made flatter, they are more to sensitive positioning error. Due to a lack of precise instruments, data, and information, it can be difficult to surgically arrange for the rotational positioning of an anatomic patellar component so that it precisely matches the orientation of the patello-femoral groove geometry on the femoral component. In other words, flatter components are more sensitive to mal-rotation. When non-axis-symmetric patellar components are not implanted at the proper position, it can cause ever greater wear concerns than the axis-symmetric designs discussed above.
For example, some designs provide components having saddle shaped articulating areas separated by a flat ridge. While these components theoretically allow for a larger surface contact area between the patellar component and the femoral component, they are also extremely sensitive to alignment (particularly rotational alignment and the inclination of the patellar component to the natural patellar bone). When surgical accuracy is not absolute, the error in alignment can cause edge loading and excessive fixation loading of the patellar component.
Another problem experienced by conforming, anatomically-shaped patellas (such as saddle-shaped patellas) is that they have different constraint patterns with the femoral component throughout the range of flexion. Specifically, the constraint of the patella riding in the trochlear groove is different from the constraint of the patella riding in the intracondylar area. As a consequence, the patella seeks different equilibrium positions while articulating in each zone. During the transition of the patella from the trochlear groove to the intracondylar area, there is often a readjustment translation and/or rotation movement caused by the patella arriving at a new equilibrium condition with the different constraint pattern. This movement is referred to as clunk, and it is especially apparent during extension from deep flexion. As previously mentioned, it can cause pain and may also be perceived by the patient as instability as the transition may be non-linear and possibly inconsistent from one area to another.
Accordingly, there is a need for a patellar component design that is more optimally shaped so that it can help reduce shear force, but that is also shaped to accommodate slight implantation error. There is also a need for a patellar component that can help lessen anterior knee pain, particularly during deep-flexion activities because the component/bone interface shear force is so high during these activities. There is also a need for a patellar component that can transition during the range of knee movement in a controlled way. There is a further need for a patellar component that has an increased volume of material in certain regions that strengthen the component and help reduce failures.